1. Field of the Invention
This invention pertains to underground storage tanks, such as those conventionally employed for the storage and dispensing of gasoline at automobile service stations. More specifically, it pertains to an underground storage tank made of corrosion-resistant materials provided with an internal steel rib.
2. Background of the Prior Art
A wide variety of liquid materials, notably among them petroleum distillate products, are kept in underground storage tanks, for controlled release or dispensing, generally through aboveground dispensing means, such as service station pumps and the like. Conventionally, such tanks are cylindrical in shape, with dome or egg-shaped end caps at either end of the cylinder. Such tanks are buried in the ground, with access to a fill pipe, and a pump for removing liquid from the storage tank.
Recently, considerable attention has been focused on such storage tanks, as potential sources of environmental pollution. In particular, concern has been raised that leakage from, or failure of, the tank, can result in the undetected release of a large quantity of hazardous product in densely populated areas. As one example of potential problems, the use of steel tanks is presented with the possibility of corrosion of the steel, particularly if the underground installation site should become filled with water, or brine. Thus, as set forth in U.S. Pat. Nos. 3,335,904 and 3,700,512, there has been increasing stress placed on the use of corrosion-resistant materials, such as resin reinforced with filamentary materials, generally fiberglass, in substitution of steel. While such materials may be generally designed stronger than steel, nonetheless, there remains in the marketplace a great desire for steel tanks, where possible, perhaps because of the common familiarity with the strength of such products. Additionally, steel alloys, on a volume basis, present a stiffer material than conventional reinforced resins, and accordingly may offer superior compression resistance, inch-for-inch of material. Compression of a tank in a "wet hole" represents the "worst possible scenario", in terms of design tolerances. To this end, many steel tanks are treated with corrosion-preventive coatings, yet this remains a partial solution, at best.
Even the use of fiberglass reinforced resin materials, etc., however, is not a perfect solution. In particular, the problem of leakage presented by damage to, or puncture of, the tank, remains. To this end, a variety of double-walled tanks, made of corrosion-resistant materials, have been advanced. Representative of such designs is that described in U.S. Pat. No. 4,561,292. Such tanks actually are comprised of two concentric tanks, with an annular space therebetween. The outer tank serves as a containment means for any leakage from the inner tank, and the space between the two tanks can be provided with a monitoring means, to detect the presence of fluid of a particular type. Thus, leakage in either the outer tank or the inner tank may be detected, and addressed. Such tanks are complicated by the general and common structure of fiberglass reinforced resin tanks and the like, which employ molded-in external ribs to enhance strength, and resistance to deflection. The ribs consume a large quantity of time and material. The formation or a rib is a particularly complicated matter, whether prepared by a male molding process, as set forth at Column 2 of U.S. Pat. No. 4,561,292, or female molded, such as that disclosed in U.S. Pat. No. 4,363,687. Further, when prepared according to the former process, the ribs are not integral with the tank cylinder but attached thereto. Under the compression forces applied in a wet hole, such ribs may be "blown off", or separated from the tank leading to potential catastrophic failure.
Accordingly, it remains an object of the art to provide an underground storage tank which is contained against leakage, comprised of corrosion-resistant materials, yet retains the features and commercial attractiveness and stiffness of steel materials.